The Coulomb Force Law states that any two charged particles

This looks a lot like the Law of Universal Gravitation, which deals with attraction between objects with mass. The big difference is that while any two masses experience mutual *attraction*, two charges can either attract or repel each other, depending on whether the signs of their charges are alike:

Like gravitational (and all other) forces, Coulomb forces add as vectors. Thus to find the force on a charge from an arrangement of charges, one needs to find the vector sum of the force from each charge in the arrangement.

#### Example

Two negatively charged spheres (one with

Consider the diagram above; here

Now we can cancel. The charge of the electron cancels. The constant

Given this ratio, we know that the electron is twice as far from the large charge (

### Interactive Simulation

Electric Ice Sheet (CK-12 Simulation)

### Review

- A suspended pith ball possessing
+10 μC of charge is placed0.02m away from a metal plate possessing−6 μC of charge.- Are these objects attracted or repulsed?
- What is the force on the negatively charged object?
- What is the force on the positively charged object?

- Consider the hydrogen atom. Does the electron orbit the proton due to the force of gravity or the electric force? Calculate both forces and compare them. (You may need to look up the properties of the hydrogen atom to complete this problem.)
- Find the direction and magnitude of the force on the charge at the origin (see picture). The object at the origin has a charge of
8 μC , the object at coordinates(−2m, 0) has a charge of12 μC , and the object at coordinates(0,−4m) has a charge of44 μC . All distance units are in meters. - Two pith balls of equal and like charges are repulsed from each other as shown in the figure below. They both have a mass of
2g and are separated by30∘ . One is hanging freely from a0.5m string, while the other, also hanging from a0.5m string, is stuck like putty to the wall.- Draw the free body diagram for the hanging pith ball
- Find the distance between the leftmost pith ball and the wall (this will involve working a geometry problem)
- Find the tension in the string (Hint: use
y− direction force balance) - Find the amount of charge on the pith balls (Hint: use
x− direction force balance)

**Review (Answers)**

- a. attracted b.
1350N c.1350N Fg=1.0×10−47N andFe=2.3×10−8N . The electric force is39 orders of magnitudes bigger.0.293N and at42.5∘ - b.
0.25m c.FT=0.022N d.0.37μC